1. Field of the Invention
The present invention relates to a porous polytetrafluoroethylene tubing and, more particularly, it relates to a porous polytetrafluoroethylene tubing having a composite fibrous structure in which the outer surface portion differs from the inner surface portion in structure. Moreover, the invention also relates to a process of producing such a polytetrafluoroethylene tubing. 2. Description of the Prior Art
Porous polytetrafluorethylene tubings are known to be produced by processes as described in, for example, Japanese Patent Publication No. 13560/67 and U.S. Pat. No. 3,953,566. The features of these processes are as follows: That is, an unsintered polytetrafluoroethylene mixture containing a liquid lubricant is shaped into a sheet, rod, or tube by extrusion, rolling, or a combination of these techniques and the shaped material thus formed is stretched in at least one direction in the unsintered state, heated to a temperature higher than about 327.degree. C in a stretched state.
The structures of the porous products obtained with these known processes may differ to some extent according to the stretching ratio, the temperature at stretching, the stretching speed, etc., employed, but the porous products obtained have a micro-composite structure composed of nodes connected together by small fibers and the spaces surrounded by the nodes and the fibers form the fine cavities of the porous structure. In general, by increasing the stretching ratio in these known processes, it is possible to increase the length of the fibers, decrease the size of the nodes, and increase the porosity of the porous structure.
That is, it is generally known that porous fibrous articles having a comparatively simple fibrous structure such as porous sheets, porous rods, porous tubes, etc., can be produced.